1. Field of the Invention
The present invention relates to an image pickup apparatus capable of picking up a dynamic image or a still image, such as a video camera.
2. Related Background Art
In a digital color camera, in response to depression of a RELEASE button, an image in a field is exposed onto a solid-state image pickup element, such as a CCD or a CMOS sensor, for a desired time period, an image signal obtained thereby and representing a still image on a screen is converted into a digital signal and is subjected to predetermined processing, such as YC processing, and an image signal of a predetermined format is obtained. Digital image signals representing the picked up images are recorded in a semiconductor memory on a picture-by-picture (frame) basis. The recorded image signals are read as desired and reproduced as signals capable of being displayed or printed, and are output to a monitor or the like to be displayed.
One example of a technology for thinning a digital color camera is disclosed in Japanese Patent Application Laid-Open No. 10-145802. In Japanese Patent Application Laid-Open No. 10-145802, an image pickup screen is divided into a plurality of regions, and an imaging optical system is provided for each of the regions to form a partial image of an object, wherein one object image is formed with regard to one imaging optical system, and object images (the number of which corresponds to the number of the divisions of the image pickup screen) are projected onto a single image pickup element.
By arranging a plurality of small island-like image pickup regions, and by providing each of them with a small-sized imaging system, a thinner image pickup apparatus can be materialized.
Generally, making the overall area of the image pickup element larger lowers the production yield, and, due to limitation on the cost, a practical upper limit is placed on the size of the image pickup element. When, as disclosed in the Japanese Patent Application Laid-Open No. 10-145802, small island-like image pickup regions are arranged for the purpose of thinning the image pickup apparatus and obtaining a high definition image, it is necessary to make at least the pixel pitch of the image pickup element small, in accordance with a sampling theory, in order to make the spatial frequency, which can be represented by the photoelectric conversion output of the image pickup element, high.
In addition to this, an image pickup optical system for forming an object image on the image pickup element is required to have high contrast up to a higher frequency.
The imaging performance of an imaging system is represented by a response function referred to as OTF. When the above characteristics are represented by the OTF characteristics, it is required that the response curve maintains high response from a low frequency to a high frequency and that once the response is lowered to zero, the response thereafter does not have a value other than zero.
The phenomenon that the response becomes negative after it is once lowered to zero is referred to as spurious resolution, which depends on the aberration characteristics of the image pickup optical system. At a spatial frequency where the response is negative, a portion which should be black becomes white while a portion which should be white becomes black, and thereby reversal of negative/positive occurs. An imaging system which causes spurious resolution has a strong tendency to have low response in a medium frequency range. Thus, the image as a whole does not have enough contrast and detailed portions are unnaturally represented. Such an imaging system is most inappropriate for recording a person or a scene.
There are two methods of improving the aberration characteristics of an image pickup optical system to alleviate the problem of spurious resolution. One is a method of increasing the degree of freedom in designing by, for example, increasing the number of lenses forming the system, making the lenses nonspherical, using anomalous dispersion glass, or complexly using diffraction optical elements. The other is a method of narrowing the imaging light flux.
The former method, that is, the method of increasing the degree of freedom in designing results in complexity of the structure of the objective optical system. This method is therefore inappropriate for a thin image pickup apparatus.
On the other hand, the latter method, that is, the method of using a narrower light flux conforms to a thin image pickup apparatus. However, when the light flux is narrowed to a certain extent or more, another problem arises in that the contrast in a high frequency range is decreased due to diffraction of light. In this state, an image is formed including a bright spot at the center and diffraction stripes surrounding the bright spot. These are caused by relative increase in the intensity of the diffraction stripes due to peripheral waves generated on the periphery of the diaphragm opening. This can be understood also from the fact that, if the diaphragm radius is reduced by ½, the periphery of the opening is reduced by ½, while the area of the opening is reduced to ¼.
Therefore, conventionally, it is difficult to materialize a simply structured image pickup optical system which obtains a high definition image corresponding to a small pixel pitch.